Switchable capacitors are used in a variety of architectures that require transitions between states of effective capacitance. Switchable capacitors are used, for instance, to modify the tuning frequency, change the bandwidth, and perform tweak matching. Typical switchable capacitors comprise a capacitor followed by a switch. The smallest step size in these types of architectures depends on the size of the smallest capacitor that can be used.
For instance, if a 20 fempto-farad (ff) (or 10−12 Farad) capacitor is the smallest capacitor that is used, then the smallest step size is 20 fF, because the capacitors are connected together and controlled individually by the opening and the closing of switches. Therefore, some, all, or none of the capacitors can be turned on or off at any given moment and the total capacitance of the system is measured by adding together the capacitance values for all of the capacitors that are turned on.
In applications that use binary control signals to control switchable capacitors, the current technique is for bits to control the switches on the capacitors. For instance, in a system having a branch with eight capacitors, 3 bits can be used to control the 8 possible capacitor configurations. Turning the eight capacitors off in this branch is caused by the signal 000. Likewise, turning two of the capacitor off results from a 101 binary signal. An arbitrary number of capacitors can be connected in this manner to be controlled by an arbitrarily large binary signal as needed. The step size, however, cannot be made smaller than the smallest capacitor in the branch.
It is desirable in many applications to provide a small capacitance step size in a switchable capacitor system. It may be impossible to continually reduce the step merely by trying to build a smaller capacitor, because the smallest capacitor size achievable is limited by the parasitics around the capacitor. This physical limitation will make it difficult to significantly reduce the step size using conventional techniques.